Particulate zinc is a known corrosion inhibitor, widely used in industrial maintenance paints where its selective galvanic action protects substrates against corrosion.
Although these paints give excellent results, they do not satisfactorily inhibit "bleeding", which occurs when a steel substrate, coated with such a zinc-containing paint, shows exposed metal through accident or failure of the film and is exposed to chlorides.sup.1. This bleeding occurs in paints containing as much as 50%, by weight of the binder components, of zinc and is still a problem when zinc levels reach as high as 80%, by weight of the binder. FNT .sup.1. By "bleeding" I mean the exudation of rust or other oxides from the wound. This exudate can discolor the finish, and, if it occurs in a container, may contaminate the container's contents.
I have now found that bleeding is significantly reduced and that a greatly enhanced galvanic action corrosion-inhibiting effect is obtained if a water-insoluble zinc salt is used in conjunction with particulate zinc in these coating compositions. This combination of zinc and water insoluble zinc salt forms a corrosion-inhibiting system which I have found to be useful with many types of binder components. I have found the system to be especially useful in compositions designed to give finishes with great resistance to degradation on exposure to boiling water, such as those used to coat interiors of water heater tanks.